1. Field of the Invention
This invention relates to dies for blow molding thermoplastic containers, and particularly a die system comprising sets of separate die elements of modular construction which can be assembled in different combinations to provide mold cavities of correspondingly different configurations and sizes. The system provides a means of using some of the same die elements for several different molding configuration and size requirements, whereby to enable the molder to customize containers to the particular requirements of different customers at substantial savings in tooling inventory and expense.
2. Description of the Prior Art
Blow molding of thermoplastic resin containers is conventionally accomplished by extruding a parison or sleeve of the thermoplastic resin from a ring die to cause the parison to hang in the parting plane of a split die. Complementary die members or die halves are suitably mounted on movable carriers positioned below the extrusion die and are moved between a closed position, in which the die halves are in abutment along the parting plane, and an open position in which the die halves are spaced apart so as to straddle the depending parison. Generally the entire die assembly is arranged to be translated bodily from a position directly below the extrusion die to a position laterally displaced therefrom. The arrangement facilitates automated production in that when a sufficient length of parison has been extruded between the parted die halves and these have been closed, the closed die with the parison clamped therein is moved laterally past a severing tool which cuts the clamped length of parison within the die from the continuously extruding portion. This allows unimpeded continuous extrusion of the parison while molding of the severed length in the die is being completed. The process is repeated after ejecting the finished part upon opening the die halves and shifting them back to straddle the depending parison. During the molding step, fluid pressure is introduced interiorily of the clamped portion of the parison in the mold to cause the parison walls to assume the full configuration of the mold cavity.
Examples of prior blow molding systems of the type discussed above appear in U.S. Pat. Nos. 3,317,955, 3,452,125, 3,536,435, 3,659,999, 3,796,780, 3,828,969, and 3,886,645.
Extensive use of the process has been made to blow molding of smaller containers or carrying cases of both single and double wall type, as in cases for protecting and transporting small hand tools and the like. Because of tooling costs in making the necessary dies, particularly where containers of larger sizes are involved, e.g. over 100 square inches of box area, it is frequently impractical for a molder to make it economically feasible to meet a customer's order for a relatively small number of containers. In the standard method of making dies for blow molding operations, the die maker in many cases starts with large solid blocks of metal, typically of aluminum alloys, and "hogs out" or machines away large portions of the metal from the block to produce the mold cavity of a typical die half for a split mold. In other cases, the molds are cast and this requires preliminary preparation of patterns and porosity-free cast surfaces, resulting in an expensive piece of metal. Attempts by molders to provide tooling for standard or stock sizes of containers, particularly in the case of larger sizes, is difficult because of the multitude of sizes possible, and the large changes and cost increments between sizes. Therefore, unless a customer's order is sufficiently large to justify amortization of tooling design for a specific container, the molder is unable to offer the customer containers that more closely match his particular size requirements.
Some attempt has been made to provide molds having capability for substituting die parts or portions to give some flexibility of product configuration without requiring complete, separate, die sets for each different configuration. U.S. Pat. No. 3,807,928 describes blow molding apparatus having a two-part die for molding plastic bottles, wherein the die is divided into a neck portion and a body portion so that different "finish" or neck configurations can be provided on the same or different body portions. That system, however, does not offer a solution to the problem of requiring completely separate die sets for each different body size. The body-defining portion of the set generally constitutes the major part of the die; furthermore machining or casting of the die half defining the body cavity is the major factor in tooling expense and time of preparation. The prior art has therefore not provided an answer to those problems to which this invention is more particularly directed.